x86: Add missing IS_IN (libc) check to memmove-ssse3.S

x86: Add missing IS_IN (libc) check to memmove-ssse3.S

Was missing to for the multiarch build rtld-memmove-ssse3.os was
being built and exporting symbols:

>$ nm string/rtld-memmove-ssse3.os
                 U __GI___chk_fail
0000000000000020 T __memcpy_chk_ssse3
0000000000000040 T __memcpy_ssse3
0000000000000020 T __memmove_chk_ssse3
0000000000000040 T __memmove_ssse3
0000000000000000 T __mempcpy_chk_ssse3
0000000000000010 T __mempcpy_ssse3
                 U __x86_shared_cache_size_half

Introduced after 2.35 in:

commit 26b2478322db94edc9e0e8f577b2f71d291e5acb
Author: Noah Goldstein <goldstein.w.n@gmail.com>
Date:   Thu Apr 14 11:47:40 2022 -0500

    x86: Reduce code size of mem{move|pcpy|cpy}-ssse3